Improve default sshd_config, make configuring PKI authentication gentler on new installations

Sum of changes:

* Create `/usr/config/.ssh/`
* Add (or overwrite existing) file `/usr/config/.ssh/authorized_keys` with an empty string to assure users that no default ssh keys are being shipped
* Enable `PubkeyAuthentication` in sshd_config by default
* Add a note providing the user instructions on how to disable password authentication through ssh if they desire, seeing as there's no way to change the root password without rebuilding openelec itself.


***Note:*** I am assuming here that `/usr/config/` is copied on first boot to `/storage/.config/`. If this is not the case, please let me know.